The present invention relates to a photosensitive composition comprising a binder which is insoluble in water and soluble in aqueous-alkaline solutions, a photosensitive 1,2-quinonediazide or a photosensitive mixture comprising a compound which splits off acid upon exposure to light and a compound containing at least one acid-cleavable C-O-C group, and a symmetric triazine-alkyl(aryl)-ether as a substance which promotes thermal hardening. The invention also relates to a copying material which is prepared from a layer support and a photosensitive layer comprising said composition and to processes for the production of negative relief copies by exposing, heating, overall exposing, developing and optionally baking a normally positive-working copying material.
Positive-working photosensitive compositions, the image-wise exposed areas of which become soluble in a developer solution and the unexposed areas of which remain insoluble, are known in the art. For the preparation of such materials, layers containing o-quinonediazides as photosensitive compounds and which additionally contain resins with groups providing alkaline solubility, for example phenolic resins, are primarily used in practice.
Photosensitive compositions based on acidcleavable compounds are also known. They contain orthocarboxylic acid derivatives, monomeric or polymeric acetals, enol ethers or acylimino carbonates as the acid-cleavable compounds. The radiation-sensitive compounds splitting off acid that are contained in these compositions predominantly comprise organic halogen compounds, in particular, s-triazines substituted by methyl halide groups. The binders used comprise resins with groups providing alkaline solubility.
It is also known that a normally positive working copying material based on o-quinonediazides can be used as a negative-working copying material, in the presence of appropriate additives and by a particular sequence of treatments British Patent Application No. 2 082 339 describes a reversal process of that kind, in which a photosensitive mixture comprising an o-quinonediazide and at least one resol is employed as a negative-working recording material German Offenlegungsschrift No. 25 29 054 (corresponding to U.S. Pat. No. 4,104,070) describes resist layers based on 1,2-quinonediazides for use in a reversal process. These layers contain a hydroxyethylimidazoline as an additive. A similar material containing secondary or tertiary amines is disclosed in U.S. Pat. No. 4,196,003. European Patent Application No. 0 133 216 (corresponding to U.S. Pat. No. 4,581,321) describes a reversal process in which the o-quinonediazide-containing copying layer contains an additive comprising a hexamethylolmelamine ether which is preferably used as a crosslinking agent in polymer reactions, while European Patent Application No. 0 131 238 (corresponding to U.S. Pat. No. 4,576,901) discloses a reversal process in which the photosensitive material does not require any of the above indicated basic or acidic additives.
There has also been disclosed a positive negative reversal process based on acid-cleavable compounds instead of 1,2-quinonediazides, as described in European Patent Application No. 0 082 463 (corresponding to U.S. Pat. No. 4,506,006).
The known reversal processes basically comprise the same sequence of processing steps, including heating the printing plate after image-wise exposure, re-exposing the cooled plate without an original and then developing by means of an aqueous alkaline developer.
The prior art reversal processes are based on the fact that the photodecomposition products produced form an insoluble reaction product when they are heated. This thermal hardening preferably takes place in the presence of particular basic or acidic additives in the copying layer or in the presence of particular crosslinking agents having multifunctional reactive groups.
Additives of this kind, however, usually have an adverse influence on the shelf life of the copying material and on particular copying properties, for example, photosensitivity and image contrast after exposure. In addition, for many applications the temperature required for image reversal is undesirably high, the duration of heating at a relatively low temperature is too long, or the temperature range for the reversal process is too narrow.
In the reversal process described in European Patent Application No. 0 133 216, the copying layer contains, in addition to a 1,2-quinonediazide, an ether of hexamethylolmelamine as an essential constituent. Although such melamine ethers advantageously promote the image reversal in the exposed images areas during thermal treatment of a normally positive-working photosensitive material, they, nevertheless, have the disadvantage that the temperature tolerances of the reversal range are insufficient, which may give rise to varying copying results. It is also a disadvantage that, due to the presence of the reactive ether groups, the printing plates have a poor shelf life, in particular at elevated temperatures, which leads to an undesirable premature crosslinking and adversely influences the usability of the plates.
German Patent Application, File No. P 37 11 263.5 of Apr. 3, 1987, (corresponding to U.S. appln. Ser. No. 177,411, filed Apr. 4, 1988) has proposed a both positive and negative-working photosensitive composition containing, as the substance promoting thermal hardening, a symmetric triazine-alkyl(aryl)ether which may carry a dialkylamino group on a carbon atom. It is, however, a disadvantage that, due to the strong basicity of this moiety, the acid produced upon exposure in the presence of appropriate photochemical acid formers is no longer sufficient for obtaining a color change in the exposed layer areas of the copying layer dyed with an indicator dye. In practical application, this lack in image contrast leads to faulty copies, in particular, when exposure is carried out in so-called step-and-repeat machines, in which a single diapositive, for example, for the printing of labels, is repeatedly projected, side by side, onto the printing plate. Since, after exposure, there is only a weak contrast between image and non-image areas on the printing plate, especially when viewing the plate in yellow light, as is necessary in the processing of photosensitive printing plates, an accurate and space-saving positioning of the diapositive may be very difficult. As against this, 2,4,6-triphenoxy-s-triazine which has also been proposed possesses a lower basicity and thus does not have the above-described adverse influence on the image contrast resulting after exposure, but it has the disadvantage that it is relatively sparingly soluble in the organic solvents customarily used for preparing coating solutions. Upon drying of the coating solution applied to the support this compound is thus prematurely precipitated and, as a result, a non-uniform and, consequently, inadequate coating quality is obtained.